I believe that unit is like the GP-334 by Motorola.
I checked www.mods.dk
and they have some information on Motorolas, you should check on the GP-300 manual they have, and maybe it will give you some idea.
I also tried Google and it gave some sites that the server will translate, and you might get lucky and find a European dealer who has some old manuals.
Good luck, 73 Ron